Incandescent lamps have been well known for several decades and have been employed in a wide variety of circumstances. In more commonplace applications the lamps have been optimized around considerations of fire safety, heat load, size, etc. In the medical field, the design of instruments which use light sources are evolving along with the light sources they employ. Unlike the commonplace applications of incandescent lamps, medical applications require a different set of considerations. For example, in a commonplace application where a higher intensity of light is required, the designer might increase the voltage of the lamp and its glass envelope size. However, in medical applications, increased voltage and current, and even lamp size may not be compatible with the medical instrument with which the light is used.
One method for increasing the light intensity has been the use of fiber optics. In this method, a very high intensity light source is placed in optical alignment with one end of a fiber optic cable. The cable is then extended into a medical instrument to supply light where needed. The disadvantages to this method are the excessive heat generated at the light source, the inefficient light capture since only a portion of the light ever makes its way into the fiber optic cable, and the limited mobility of the medical instrument, since it must always be at the end of a fiber optic cable. Sterilization requirements may also mitigate against the use of fiber optics at the working end of an instrument since repeated sterilization may cause a denigration of the fiber.
In portable instruments, several limitations are present. First, efficiency is of paramount importance. Since most of the instruments are battery powered, a large current drain would be unacceptable. An extended use would require an interruption in the procedure to change batteries, even if new batteries were supplied at the beginning of each new procedure.
Another important aspect is safety. In medical instruments, the bulb must be adequately supported and protected from breakage. Not only would breakage interrupt the procedure, but shards of glass could be introduced into contact with the patient. The bulb could be encased in an additional layer of translucent material, but such would cause a degradation of performance. This is especially true for white light having multiple frequencies which cannot be wavelength matched across a given thickness of material. Added covering materials would increase the heat load, diminish the light transmission, and would require more power for a given level of output. Greater power would, in turn, shorten the life of the incandescent bulb.
Recessing the bulb within a protective sheath for protection causes other problems. First, most of the light from the bulb which impinges on the walls of the sheath will otherwise be lost. Next, once the bulb is inserted within a protective sheath, it may be physically difficult to remove it from the sheath. Bulbs having protective metal envelopes, including bulbs with screw bases and which use the metal envelopes for reflectors are not able to gain sufficient structural stability from the metal envelopes. A sharp blow to the metal envelope could produce sufficient bending moment in the bulb to cause it to snap. Bringing the metal envelope closer to and in a supportive relationship with the bulb can defeat the reflector action of the metal envelope.
Another important issue is cost and reproducibility. For a given set of constraints, an incandescent light system could be custom designed. However, the driving force behind the lighting industry is mass production and cost. The incandescent system should be amenable to cost effective mass production such that the cost of a light source to be used with any instrument should be virtually insignificant compared to the cost of the overall instrument. As such, a system should have good integrity, meet the requirements of the sensitive environment, typically a medical environment, and be easily and cost effectively produced in large numbers.
What is therefore needed is a system for using high intensity bulbs, especially in environment sensitive applications such as medical applications. Such a system should be efficient, producing significant light output without significant loss of light not directed into the area of interest. Such a system should be light weight and very protective of the filament containing glass envelope but without significantly increasing the effort and speed with which a burned-out bulb may be replaced.